Fix Schemeful same-site context downgrade UMAs

Correct the histogram names for the schemeful same-site context
downgrade UMAs by adding "Cookie."

Bug: 1085517
Change-Id: Id3f46ad118a24e37123bb95c7876ab464f550fc7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212842Reviewed-by: Lily Chen <chlily@chromium.org>
Commit-Queue: Steven Bingler <bingler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771239}

net/cookies/canonical_cookie.cc

@@ -684,7 +684,7 @@ CookieInclusionStatus CanonicalCookie::IncludeForRequestURL(
   }
 
   if (status.ShouldRecordDowngradeMetrics()) {
-    UMA_HISTOGRAM_ENUMERATION("SameSiteContextDowngradeRequest",
+    UMA_HISTOGRAM_ENUMERATION("Cookie.SameSiteContextDowngradeRequest",
                               status.GetBreakingDowngradeMetricsEnumValue(url));
   }
 

net/cookies/cookie_monster.cc

@@ -1205,7 +1205,7 @@ void CookieMonster::SetCanonicalCookie(std::unique_ptr<CanonicalCookie> cc,
   // cookie warnings/exclusions, record the downgrade metric.
   if (status.ShouldRecordDowngradeMetrics()) {
     UMA_HISTOGRAM_ENUMERATION(
-        "SameSiteContextDowngradeResponse",
+        "Cookie.SameSiteContextDowngradeResponse",
         status.GetBreakingDowngradeMetricsEnumValue(source_url));
   }